Forbidden {Sirius x Trans! OC}

Flynn Weasley is Percy's twin. He's the so-called 'black sheep' of the family. He was the first born daughter of the family, but he came out as trans when he and Percy were in second year. In his seventh year, they got two new DADA professors. Professor Lupin and Professor Black. When he and Professor Black take a liking to each other, what will happen?
There will be multiple people who use his dead name, either because I don't like them or because I just feel like that's just who they are.
Set during prisoner of Azkaban, but Sirius was found innocent, and Peter is the escaped murderer. Instead of Scabers, Ron has a black cat named Shadow.